198 



ORGANOGRAPHY. 



Fig. 390. Fig. 391. in Avhich the pedicels are very- 



short, so as not to be clearly dis- 

 tinguishable. Examples of it 

 may be seen in the Rib-grass 

 (Plantago) (Jig. 390), and Ver- 

 vain ( Verbena officinalis) (Jig. 

 391). In this kind of inflores- 

 cence it will be observed that 

 the flowers at the lower part of 

 the spike are in fruit, while 

 those near the middle are in 

 full flower, and those at the 

 top are still undeveloped. The 

 flowers here therefore, open first 

 at the base, and last at the 

 apex. Such a mode of opening 

 is called centripetal. This mode 

 of expansion is universal in the 

 different kinds of indefinite in- 

 florescence, which in all cases 

 open from the base to the apex 

 if the axis is elongated (Jig. 39 1 ), 

 or from the circumference to- 

 wards the centre if it is de- 

 pressed or dilated (Jig. 405). 

 This centripetal order of ex- 

 pansion necessarily arises from 

 the mode of development of 

 such kinds of inflorescence ; 

 thus, the flower-buds which are 

 situated at the base, are those 

 which arc first formed and which 

 arc consequently the oldest ; as 

 the axis elongates upwards, it 

 is continually producing other 

 flower-buds, the age of which continues to decrease as we 

 approach the growing point or apex ; and as flower-buds are 

 necessarily most developed in the order of their age, it follows 

 that those at the base will open first, and that the order of 

 expansion will proceed gradually upwards towards tlie apex, 

 or centripetdlhj. Hence all indefinite inflorescences have a cen- 

 tripetal order of expansion. 



There are five otlier kinds of indefinite inflorescence which 

 are simply modifications of the spike. These arc the Amentum 

 or Catkin, the Spadix, the Ix)custa, tlie Cone, and tlic tStrobile. 



b. The Amentum or Catkin.— This is a kind of spike which only 

 bears barren flowers — that is, only stamens (Jig. 393), or jjistils 

 (Jig. 392); tliese are separated from each other by squamous 



Fig. 300. Spike of a species of Rib- 

 grass (Plnntaoo). Fig. 891. 



Spike of Vervain ( Verbena). Tiie 

 flowers at the base liave already 

 passed into tlie state of fruit, 

 wliilst those at the apex are still 

 unexpanded. 



